Vitamin D, a fat-soluble vitamin, has two major forms; D2 (ergocalciferol) and D3 (cholecalciferol). Since vertebrates synthesize vitamin D3 from sunshine, researchers wanted to test for any biological differences between vitamin D2 and D3 once inside the body.

In one study, 33 healthy adults took 50,000 IU of vitamin D2 or D3 per week. After 12 weeks, blood serum levels of the active form of vitamin D (calcitriol) were 70 percent higher in the vitamin D3 group than in the vitamin D2 group. The vitamin D3 group also had two to three times the amount of stored vitamin D.

Study authors said that most North American pharmaceutical preparations use vitamin D2, and recommend switching to vitamin D3 for its greater potency and lower cost.

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